In video coding, block information of neighboring blocks are usually utilized to predict syntax elements, such as motion data, of a current block. The neighboring blocks usually correspond to blocks previously processed so that the information of the neighboring blocks can be derived at the decoder side without side information or with minimum side information. The correlation among an underlying block and neighboring blocks may be utilized to improve the coding efficiency of Entropy Coding (EC). To exploit the correlation among the underlying block and neighboring blocks, context-based adaptive entropy coding (also called context adaptive entropy coding) has been used extensively in newer video coding systems such as H.264 and the emerging High Efficiency Video Coding (HEVC) system. There are two types of EC modes. One mode is Context-based Adaptive Binary Arithmetic Coding (CABAC), and the other mode is Context-based Adaptive Variable Length Coding (CAVLC). In both CABAC and CAVLC, information from neighboring blocks is often used as contexts for entropy coding a syntax element of the current block.
The neighboring blocks may be these blocks on the upper side and/or left side of the current block. When the blocks in a picture are processed in a raster scan order, a line buffer may be required to store information associated with blocks in a previous row. For example, a processing unit used in HEVC Test Model version 3.0 (HM-3.0) may correspond to a Coding Unit (CU), a Prediction Unit (PU), a Transform Unit (TU), or a Largest CU (LCU). In the case of CAVLC coding of motion vector difference (MVD) of the current block, it requires the MVs of the left block and the upper block for context formation. Therefore, the information associated with motion vectors for the blocks in a previous row of LCUs has to be buffered if the processing order is LCU row by LCU row. In HEVC, CABAC and CAVLC are used to encode various syntax elements associated with the blocks. The line buffer size may be very large, especially for large-size pictures. Therefore, it is desirable to develop method and apparatus to reduce the line buffer requirement for neighboring-block-dependent processing such as CABAC and CAVLC.